Fix testing of Supabase Anonymous Sign-Ins: Include is_anonymous field of user on jwt for authenticate_as functions

[molti-tasking]

First of all: Great package, thanks you so much!! I have built many, many tests with these helpers 😃

When testing anonymous users in supabase the RLS policies rely on the is_anonymous field in the jwt:

select (auth.jwt()->>'is_anonymous')::boolean

Please, refer to the supabase docs for more information on anonymous users: https://supabase.com/docs/guides/auth/auth-anonymous?queryGroups=language&language=js#access-control

In the end the problem is in the "authenticate_as" function, because there are some fields mapped to the jwt claims, but the is_anonymous field is missing. Adding it should solve this issue and the supabase-test-helpers should be sufficient for supabase RLS testing.